Natural adhesives such as animal glues, fish glues, vegetable glues and casein (the main protein in milk) are generally set upon solvent preparation and offer low strength and are susceptible to moisture and mold. Their use is mainly for joining low strength materials.
Elastomer adhesives such as natural rubbers, neoprene, acrylonitride budadiene, butyl/rubber adhesives, styrene butadiene rubber adhesives, polyurethane adhesives, polysulphide rubber adhesives, and silicone rubber adhesives are based on natural and synthetic rubbers set by solvent evaporation or heat curing; they have relatively low strength and suffer from creep and therefore are not usually used for stressed joints. More typically, they are used for flexible bonding of plastics and rubbers.
Thermoplastic adhesives such as polyvinyl acetate (PVA), polyvinyl alcohol (PVA), polyacrylates, polyster acrylics, acrylic solvent cement, cyananoacrylates (superglue), silicone resins, polyamides and acrylic acid diesters have low/medium strength and may suffer from creep and attack from water but not from oils.
Thermoset adhesives such as phenol formaldehyde (PF) resins, phenolic neoprene, polysters, polymides and epoxy resins set as a result of the buildup of rigid molecular chains with cross-linking.
Toughened rubber modified adhesives include small rubber-like particles dispersed throughout a glassy matrix are resistant to crack propagation and have been applied to acrylic and epoxy-based adhesives.
The general performance characteristics of these adhesives in terms of shear strength and range of operating temperatures are summarized in the table below.
TABLE 1Properties of conventional adhesivesShearOperatingstrengthtemperature(MPa)(C.)Adhesive TypeMin.Max.Min.Max.Rubber0.353.5−20150PVA (white glue)1.46.9Cyanoacrilate6.913.880Anaerpbo6.913.8200Polyurethane6.917.2−200150Rubber modified epoxy13.824.1−4090Epoxy10.327.8200Polymide13.827.6350Rubber modified epoxy20.741.4180Note:1 MPa = 10 bar about 147 psi; 1,000 psi = about 6.9 MPa
The strength of adhesive joint is dependent on how well the adhesive has bonded to the surface of a material (i.e., substrate), as well as on the cohesive strength of the adhesive itself. It is noted from Table 1, that, for the most part, the maximum bonding strength attainable from the range of conventional adhesives is about 28 MPa (4,000 psi). Virtually all of them use ingredients which emit or themselves emit volatile organic compounds (VOCs) and they require set times ranging from minutes to tens of minutes to days, and the operating temperatures are generally below 150-200° C.
Limitations on adhesives that are currently known in the marketplace include their lability in the presence of solvents like acetone and water. The surfaces they join often require careful surface preparation such as degreasing with steam, organic solvents or alkaline cleaners; mechanical methods applications such as blasting, brushing, or grinding; dipping in boiling water after etching in caustic soda; yellow or green chromate treatment; anodizing in chromic, phosphoric or sulphuric acid. Many adhesives require long times to set before allowing use of the joined substrates (e.g., Gorilla Glue asks for 24 h post-application).
Many materials are difficult to bond with adhesives and require specialized surface preparation. This is the case for substrates such as teflon (PTFE), marble aluminum (and alloys); copper (and alloys); magnesium (and alloys); nylon, polyacetal, polyethelene, polypropylene, PVC, zinc; and glass. Such substrates often require use of primers.
Commonly, adhesive-based joints are not as strong or reliable as a fastened joint. This is especially true for susceptibility to peel loads, although simple methods such as use of double lap joints can be deployed to ameliorate such limitations. In addition, the temperature range for application can be much lower than that for fastened joints.
Most adhesives present toxicity issues due to volatile organic compounds (VOCs) and the presence of harmful chemicals such as formaldehyde. Such compounds are often used to provide for liquid state handling of adhesives during processing at room temperature.
New adhesives are needed that have improved fatigue behavior and reduced stress concentration zones. They should be easy to use such as by controlling viscosity, allowing for high throughput uses, and have sealing capability such that the adhesive joint can seal joined materials from moisture and air. In addition they should not melt or otherwise modify the substrates they are intended to join, they should be amenable to use in joining a broad range of substrates including difficult substrates to join adhesively such as teflon, marble and aluminum substrates and they should be of minimal volume. Any new adhesives technology that can overcome some or all of the limitations mentioned above are needed. Ideally new adhesives can be used in an aqueous state without the need for volatile organics.